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content. Furthermore, Wiggs, Baird and Atherton (2004)
noted that moisture that was adhered to saltating particles
could reach 2 % (gravimetric) before any influence on the
transport rate could be detected. Saltation of dry sediment
is therefore possible across wet surfaces and Sarre (1988,
1990) found that surface moisture contents of up to 14 %
had little effect on sand already in transport. Such natural
inhomogeneity in surface moisture is not taken account of
in the static conditions of wind tunnel tests and such data
tend to show a shutdown of the saltation system at lower
average surface moisture values.
Complexities in the influence of soil moisture on the
saltation system are now being investigated on beaches.
Studies by Bauer et al. (2009), Davidson-Arnott and
Bauer (2009), Jackson and Nordstrom (1997, 1998),
Wiggs, Atherton and Baird (2004), Wiggs, Baird and
Atherton (2004) and Davidson-Arnott et al. (2008) have
all noted the spatial and temporal variability in surface
moisture content on beaches and have investigated the
impacts of such variability on saltation dynamics. In
particular, the sensitivity of sediment entrainment to the
moisture status of a very thin layer of surface sediment
has been explored. Once these surface grains have dried
to a sufficient extent to allow entrainment the damper
grains below are revealed and critical entrainment
thresholds rise once again. This high temporal dynamism
in entrainment thresholds, coupled with variation in
wind speeds, produces a highly intermittent saltation
system where erosion of surface sediment at a specific
location can occur at a range of wind speeds within a very
short space of time. Davidson-Arnott and Bauer (2009)
therefore suggest that there exists a range of entrainment
thresholds, rather than a single fixed value based on an
average moisture content and grain size. Such sensitivity
of the saltation system to the moisture status of the top
few grains results in the dynamics of the system being
readily influenced by air humidity. McKenna-Neuman
and Sanderson (2008) and McKenna-Neuman (2003,
2004) note that colder airflows support much higher sand
transport rates than warmer air. They partly explain this
by the decreased adsorption of moisture from the air to
surface grains and hence reduced interparticle cohesion
and lower entrainment thresholds in the cold air case.
With the requirement for higher spatial and temporal
resolution measurements of the moisture status of surface
sediment new methods of measurement are being explored
that do not rely on gravimetric analysis of grab samples.
Such grab samples may include substantial amounts of
subsurface (and wetter) sediment. One new method re-
ceiving attention is that of measurement of surface bright-
ness using calibrated digital photography, which has been
24
20
In k b / k o
16
12
8
4
-4
-8
30
20
10
0
10
20
30
Upslope
Downslope
Figure 18.15 The effect of bedslope on sand transport rate:
solid line = geometric relationship (Bagnold, 1941), circles =
experimental data (from Hardisty and Whitehouse, 1988).
(Wiggs, Baird and Atherton, 2004; Bauer et al. , 2009).
However, its influence is complex, variable and not well
understood. The majority of our knowledge comes from
wind tunnel investigations (e.g. Cornelis and Gabriels,
2003; Han et al. , 2009; McKenna-Neuman and Nickling,
1989), although recent field studies on beaches have de-
veloped our comprehension of the complexities involved
(Bauer et al. , 2009; Davidson-Arnott and Bauer, 2009;
Wiggs, Baird and Atherton, 2004). The theoretical basis
for available predictive models is that the critical shear ve-
locity required for entrainment increases as a function of
the increased surface tension associated with pore mois-
ture. However, as shown in Figure 18.16, there is much
disagreement as to the precise mechanism by which this
should be applied. In this figure the Hotta et al. (1984)
and Belly (1964) data are based on empirical wind tun-
nel measurements while those of Kawata and Tsuchiya
(1976) are theoretical.
The data shown in Figure 18.16 suggest that the entrain-
ment system is perhaps very sensitive to changes in mois-
ture status in the range 0-4 %. However, in field exper-
iments conducted by Wiggs, Baird and Atherton (2004)
significant limitation on sediment entrainment was found
only at higher moisture values (4-6 %). They argued that
this was due to spatial inhomogeneity in the wetness of
surface sediments such that drier sediment on topographic
highs (such as crests of ripples) could be entrained and
 
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